1. Field of the Invention
The present invention relates to power distribution systems and, more particularly, to control circuits for distribution panels powered from transfer mechanisms, such as, for example, transfer switches. The invention also relates to control systems for power distribution systems.
2. Background Information
Alternate power sources are provided for any number of applications, which cannot withstand a lengthy interruption in electric power. Typically, power is provided from a primary source with back-up power provided by a secondary source. Often, the primary source is a utility and the secondary source is an auxiliary power source, such as an engine driven generator or a second utility source. The transfers between the two power sources can be made automatically or manually.
In the case of a generator driven auxiliary power source, power must be stabilized before the transfer can be made to the secondary source. In any event, the two power sources cannot be connected to the load simultaneously unless they are in phase. Thus, an open transition transfer may be employed in which the previously connected source is disconnected from the load before the other source is connected.
Transfer switches are known in the art. Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. Applications for transfer switches include stand-by applications, among others, in which the auxiliary power supply stands-by if the normal power supply should fail. Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls connection of the utility lines and the diesel generator to the facility load buses. In many installations, the transfer switch automatically starts the standby generator and connects it to the load bus upon loss of utility power, and reconnects the utility power to the load bus if utility power is reestablished.
Transfer switches typically affect an open transition between the power sources, that is, one is disconnected from the load bus before the other one is connected. Other transfer switches provide a closed transition wherein the oncoming source is connected to the load bus before the other is disconnected, in order that both power sources are connected in parallel during the transition.
Transfer switches commonly used to connect alternate power sources to a load, including networks, utilize a pair of switches each connecting one of the sources to the load. In order to prevent connecting unsynchronized sources together, the operation of the two switches is coordinated, typically by a mechanical interlock, in order that only one switch at a time can be turned on. A transfer switch typically comprises a pair of circuit interrupters combined with a drive input and a linkage system. See, for example, U.S. Pat. Nos. 5,081,367; 4,760,278; and 4,398,097.
U.S. Pat. No. 6,067,482 discloses a controller coupled with six compressors in a refrigeration system. Transfer switches are associated with each of the compressors and are selectively coupled by the controller with a utility or a generator/fuel cell. Each of the transfer switches is operable between a first position in which its associated load is connected to the utility power source and disconnected from the generator, and a second position in which its load is connected to the generator and disconnected from the utility. A processor includes two modes of operation. A first mode is implemented when the cost of locally generated energy is lower than the utility, and demand is shifted to the generator up to the maximum generator capacity. The second mode is in effect when the utility's energy rates are lower than the local energy cost, and demand is shifted to the generator only for peak shaving purposes. In the first mode, both maximum energy and peak power are shifted from the utility to the generator. In the second mode, maximum peak power and minimum energy are shifted from the utility to the generator. The processor is adapted to select a preferred combination of loads regardless of whether the transfer switches have a neutral position.
U.S. Pat. No. 6,191,500 discloses that a critical load includes those loads whose operation must be continuous. Such loads may include computer systems, communication systems and life support systems. A conventional uninterruptible power supply (UPS) system may include other loads as well as the critical load. These other loads may include building loads, HVAC loads, lighting loads, and various other loads whose operation may be desirable but not critical. These loads may be prioritized in and taken on and offline depending on power available. Typically, these loads would be online during normal operation and offline during the interruption of, for example, both utility sources.
U.S. Pat. No. 6,191,500 also discloses that the UPS system includes a plurality of UPS modules electrically coupled to at least two utility sources. Generator paralleling switchgear includes a load priority and load shedding control system that adds or reduces load on a generator bus. Each load in the UPS system has a preassigned wattage rating, which the control system employs to determine how many loads can be added as generators come onto the generator bus. As generators are randomly connected to the generator bus, the control system signals for connection of the loads in ascending sequential priority, with the highest priority load being connected first. All load-add steps are preceded by a time delay, adjustable from 0 to 3 seconds.
There is room for improvement in power distribution systems and control systems therefore.